Polyfill for dynamic module loading

importPolyfill.js

_registry = {};
importPolyfill = path => {
  if(!(path in _registry)) {
    const entry = _registry[path] = {};
    entry.promise = new Promise(resolve => entry.resolve = resolve);
    document.head.appendChild(Object.assign(
      document.createElement('script'),
      {
        type: 'module',
        innerText: `import * as X from '${path}'; _registry['${path}'].resolve(X);`,
      }
    ));
  }
  return _registry[path].promise;
}

importPolyfill.min.js

r={},importPolyfill=(a)=>{if(!(a in r)){const b=r[a]={};b.p=new Promise((c)=>b.r=c),document.head.appendChild(Object.assign(document.createElement('script'),{type:'module',innerText:`import * as X from '${a}'; r['${a}'].r(X);`}))}return r[a].p};

I added onerror support for the script tag, and escaping, here: https://gist.github.com/samthor/3ff82bd5b11314fec2e1826d4a96ce7c